A wavelength division multiplexing (WDM) scheme is a scheme for transmitting a plurality of optical signals having different wavelengths through one optical fiber. A WDM optical network includes a central office (CO), optical network units (ONUs), and an optical distribution network for interconnecting the CO and the ONUs. Among various optical distribution networks, there is the most economical technique in which a remote node (RN) including a wavelength-multi/demultiplexer is arranged in a neighboring area of optical network units, the CO and the RN are connected with each other through a single optical fiber, and the RN and the ONUs are then connected with each other through optical fibers.
Currently, a best-known technique of a WDM optical network is to separate wavelength bands for downstream optical signals and upstream optical signals and uses different wavelength bands for them. In the technique of the WDM optical network of wavelength-band separable scheme as described above, it is most typical that a circulation type wavelength-multiplexer/demultiplexer is disposed in each of the CO and the RN, the CO-wavelength-multiplexer/demultiplexer is connected with the RN-wavelength-multiplexer/demultiplexer through a single optical fiber, and respective ports of the wavelength-multiplexer/demultiplexers is then connected to ONUs. In this event, an optical transceiver includes a wavelength-band separable optical filter, an optical transmitter, and an optical receiver. The narrower the wavelength-separable band is, the more difficult it is to technically implement the wavelength-band separable optical filter. Therefore, when an upstream optical signal is used in a C-band wavelength band and a downstream optical signal is used in an L-band wavelength band, it has been frequent to employ a scheme for separating an optical module for wavelength-multiplexing the upstream optical signal and an optical module for wavelength-demultiplexing the downstream optical signal to the CO from each other.
For the conventional optical line terminal (OLT) of the WDM optical network which employs the scheme of separating the wavelength-multiplexer and wavelength-demultiplexer from each other as described above, there have been many proposals for schemes of controlling and monitoring an optical power and a wavelength of an optical transmitter of an OLT. A representative control/monitoring scheme proposes an output unit of the wavelength-multiplexer which includes an optical power branching tap coupler, a control/monitoring wavelength-demultiplexer, and a light-receiving device. To this end, it is required to add the wavelength-demultiplexer, which may increase the size and construction costs of the system. In order to overcome the demerits as described above, schemes wherein a reflection type tap coupler is put in the output unit of the wavelength-multiplexer and a control/monitoring light-receiving device is disposed before the optical transmitter have been proposed. However, these schemes also have problems in that they require a multichannel reference signal generating apparatus and a costly lock-in-amp, which complicates the system construction and increases the costs.